For volume-selective magnetic resonance spectroscopy, the spin resonance signals generated in a small zone (the selected volume) within a patient being examined are subjected to a spectral analysis. The spectra thus obtained provide information as regards the substances contained in the selected volume. Methods of this kind, including the coil systems required, are known (for example, see EP-OS 233 675 corresponding to U.S. Pat. No. 4,843,549). These coil systems comprise either a single RF coil for generating magnetic RF pulses and for receiving the spin resonance signals generated in the relevant volume, or separate RF coils for generating RF pulses on one hand and for receiving the spin resonance signals on the other hand.
As is known, examinations of this kind are performed in the presence of a strong, steady magnetic field, it being extremely important that this magnetic field is homogeneous in the volume to be examined. The deviation of the magnetic field from its mean value must be substantially less than 1 ppm throughout the volume examined.
In order to improve the homogeneity, use is made of so-called shim coils whose direct currents are individually adjusted so that overall an as homogeneous as possible field is obtained. These coils, being permanently incorporated in the MR examination apparatus, comprise an opening which is sufficiently large to accommodate a patient to be examined.
However, it has been found that the homogeneity that can be achieved by means of these shim coils is limited and that, even if the patient were introduced into an exactly homogeneous magnetic field, the magnetic field within the patient would exhibit an inhomogeneity of several ppm. This is due to strong, local variations of the magnetic susceptibility, which force the magnetic field out of the human body.